This invention relates to an apparatus for determining the need for adjustment and the adjusting of the resilience of a hollow ball having an internal pressure. More particularly, the invention is directed to a method for repressurizing and resealing tennis balls, a device for effecting said method and an apparatus for measuring a deflection characteristic of a tennis ball before, after, or during the repressurization process.
A common problem among tennis players is to have the tennis balls go "flat" before they are worn out. When flat, a tennis ball loses a portion of its original internal pressure which was manufactured into the ball. Loss of this internal pressure causes decay in the resilience or bounce characteristics and makes the tennis ball undesirable for further play.
The dynamic characteristics of a tennis ball are dependent on various factors including its internal and external geometry, the character of its construction materials, the type of medium and the degree of internal pressurization. These fundamental parameters manifest themselves to the tennis player in the form of general appearance, color, texture, weight, resistance to spin, aerodynamic properties and bounce or resilience characteristics. The tennis player sees or feels all of these characteristics as he plays the game.
There is a predictable relationship between the bounce or resilience characteristic and the static load deflection characteristic of a tennis ball. This predictable relationship exists because the United States Tennis Association performance specifications require that tennis balls approved for play exhibit this definite relationship. A critical and key part of the invention is to use this relationship to permit the static adjustment of a dynamic characteristic.
Tennis balls are normally manufactured beginning with a hollow internally pressurized spherical rubber ball. This ball in conjunction with an appropriate covering provides the correct combination of weight, diameter, moment of inertia and resilience in accordance with accepted customs and rules of the game. Once the proper combination of diameters and materials is built into the tennis ball, its resilience can be adjusted within the proper limits by adjusting its internal pressure with all other parameters remaining constant. Therefore, different manufacturers may achieve the bounce or resilience characteristics in different ways. It involves a combination of the internal pressure and the physical structure of the balls. Thus, it is possible to have the same bounce characteristics in two balls having different internal pressures and different physical structures within the official specifications.
The tennis player can feel both the general bounce level or resilience of all the balls being used during a particular game and the difference between each individual ball. An average tennis player will tolerate and adjust his game to general resilience levels of between 55% and 60%, provided all the balls in play are within 1% of each other. Tennis balls are normally used in matched sets of three each. If for any reason one of the balls loses part of its resilience, the player may have to discard the others in order to keep a matched set of three in the game. Tennis balls often lose their "playable resilience" because of a gradual loss of the original pressure manufactured into the ball. This loss occurs due to the leaking of the pressurization medium through the porosity of the inner rubber wall of the ball.
Prior art devices have been disclosed for the purpose of restoring internal pressure in tennis balls. However, known prior art devices include a pump mechanism or other complicated injecting means for accomplishing the desired result. U.S. Pat. No. 3,921,977 discloses a device having a complicated pump and valving arrangement for separately injecting air and sealant. Furthermore, the simultaneous determination of the potential bounce or resilience as related to the deflection characteristics of the ball is not included in any of the known devices.